Grand Central Dispatch

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Grand Central Dispatch (GCD) is a technology developed by Apple Inc. to optimize application support for systems with multi-core processors and other symmetric multiprocessing systems. It was first introduced in Mac OS X Snow Leopard and later included in iOS, watchOS, and tvOS.

Overview

Grand Central Dispatch is designed to help developers write applications that can take full advantage of the multiple cores available in modern hardware. It abstracts the complexities of managing threads and provides a simpler, more efficient way to execute concurrent code.

Key Concepts

Dispatch Queues

GCD introduces the concept of dispatch queues, which are responsible for executing tasks in a first-in, first-out (FIFO) order. There are two types of dispatch queues:

• Serial Queues: Execute one task at a time in the order they are added.
• Concurrent Queues: Execute multiple tasks simultaneously, but complete them in the order they were added.

Blocks

GCD uses blocks, which are self-contained units of work, to define tasks. Blocks are similar to closures or lambdas in other programming languages and can capture and store references to variables and constants from their surrounding context.

Dispatch Sources

Dispatch sources are a mechanism to monitor low-level system events, such as file descriptors, signals, and timers. They allow applications to respond to these events asynchronously.

Advantages

• Performance: By efficiently managing the execution of tasks across multiple cores, GCD can significantly improve the performance of applications.
• Simplicity: GCD abstracts the complexities of thread management, making it easier for developers to write concurrent code.
• Scalability: Applications using GCD can scale more effectively with the number of available cores.

Usage

Developers can use GCD by including the appropriate headers and libraries in their applications. The primary functions and types used in GCD include:

• `dispatch_async`
• `dispatch_sync`
• `dispatch_after`
• `dispatch_queue_t`
• `dispatch_block_t`

Related Technologies

• OpenCL
• Threading Building Blocks
• POSIX Threads
• Cocoa (API)

See Also

• Concurrency (computer science)
• Parallel computing
• Multithreading (computer architecture)
• Apple Inc.
• Mac OS X Snow Leopard
• iOS
• watchOS
• tvOS